METAL MATRIX COMPOSITES

Over the last 25 years, composite materials have moved from laboratory curiosity to material of choice in many high performance structural applications, particularly those which are weight critical. Most of these applications have been polymer matrix based and there is a large resource base on the mechanics and processing of these materials. However, there are some applications ( e.g. elevated temperatures ) where polymer based composites are unsuitable and other matrix materials must be considered. .

Metal Matrix Composites

Of particular interest is the behavior of ceramic fiber reinforced metals as they have excellent mechanical properties at elevated temperatures. Typical applications for these materials would be in engine components or hypersonic aircraft where reduced weight and higher operating conditions offer significant performance advantages ( increased engine efficiency, higher fuel economy, higher operating speeds, etc. ) Metal matrix composites are certainly not new, but they are far less commonly used than their polymer counterparts and consequently, less is known about their behavior. Like polymer based composites, many of the microstructural problems found in metal matrix composites are processing induced. However, the unique nature of metal matrix fabrication processes means that there are no polymer matrix analogues and new strategies for identifying and circumventing these problems must be developed.

Micromechanics Models

In this study, we plan to develop mathematical models which simulate the behavior of both continuous and discontinuous fiber reinforced metal matrix composites during fabrication. Particular attention will be devoted to describing the evolution of local microstructural features ( fiber concentration, fiber orientation, fiber bending/breaking, matrix porosity/degree of consolidation ) based on constituent material properties, initial material state and processing conditions( geometric constraints, pressure-time history, temperature-time history, etc. ). The goal here is to be able to identify initial material states and processing conditions which would result in composites with microstructural anomalies ( segregated fibers, broken fibers, misoriented fibers, matrix porosity, etc. ) which would compromise the load bearing capability of the finished component.

Model Verification

The experimental approach we propose to follow is evolutionary from simple to complex systems. Initially, we plan to study in detail the response of a single fiber reinforced system so that the mechanical response of an individual fiber can be accurately modeled. Here we will experimentally reproduce a scaled version of a single fiber, representative volume element surrounded by a region with averaged material properties, much in the manner of self consitent micromechanical models. Next we will study planar reinforced, dilute ( low volume fraction ) fiber reinforced systems where fiber-fiber interaction will be limited and orientation changes will be confined to a single plane. Next we will move into both highly filled 2D systems and dilute 3D systems. Finally, we will study the properties of filled 3D systems. In these experiments novel materials characterization techniques will be utilized to quantify the microstructural parameters of interest. For the single fiber and dilute experiments this means tracking the position ( x,y,z ) and orientation (of each fiber as well local fiber bending and matrix porosity. For the highly filled systems, tracking individual fibers will be impossible so spatially averaged properties will be used. We will use these spatially averaged properties to develop concentration and fiber orientation distributions within the composite at selected intervals throughout processing.

Project Goals

From these studies it is hoped that an improved understanding of fiber matrix interaction will be obtained. From this new insight, process models of improved accuracy and enhanced predictive capability should result. Ultimately, the objective is to develop mechanics models of the fabrication process that are sufficiently accurate and robust to be used for on-line process control of metal-matrix composite fabrication.

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